In the case of a first conventional inverter controller, according to the operation method thereof during an instantaneous power failure, an AC motor is decelerated when the terminal voltage (intermediate voltage) of a smoothing capacitor lowers to level 1 or less, the AC motor is accelerated when the intermediate voltage rises to level 2 or more due to the regenerated power obtained by deceleration, and acceleration/deceleration control is repeated subsequently depending on the increase or decrease of the intermediate voltage as described above, whereby the drop rate of the intermediate voltage is reduced (for example, refer to Patent document 1).
In the case of a second conventional inverter controller, the inverter controller starts deceleration using a power failure detection signal, calculates deceleration rate 1 on the basis of the target value and the detection value of a DC intermediate voltage so that the DC intermediate voltage becomes constant during deceleration, calculates deceleration rate 2 on the basis of the change rate of the DC intermediate voltage and controls the deceleration time by proportionally integrating the value obtained by multiplying the two deceleration rates, thereby stopping the deceleration when the DC intermediate voltage reaches the voltage before power failure detection or rises during deceleration (for example, refer to Patent document 2).
In the case of a third conventional inverter controller, the voltage value of the main circuit (corresponding to the terminal voltage value of the smoothing capacitor) rises when the output frequency is lowered and the motor is decelerated; the deceleration time of the motor is controlled depending on the detected voltage value of the main circuit such that when the voltage detection value is higher than a predetermined first voltage value, the deceleration time is lengthened, and such that when the voltage detection value reaches a second voltage value higher than the first voltage value, the deceleration is stopped (for example, refer to Patent document 3).
In the case of a fourth conventional inverter controller, when a regenerative load is applied during motor operation and the detected DC voltage (corresponding to the terminal voltage value of the smoothing capacitor) exceeds a reference value, the output frequency to be raised is obtained by multiplying the difference between the detected DC voltage value and the reference value by a constant proportional gain, and control is performed to raise the output frequency (for example, refer to Patent document 4).
FIG. 8 is a view showing the operation of the fourth conventional inverter controller for driving an AC motor when it is used, for example, under a load condition in which regenerated energy increases at the bottom dead center of a pressing machine. In the figure, f designates an output frequency, Vdc designates a DC bus voltage, Vdc0 designates a DC bus voltage stalling level, and Vdc2 designates an overvoltage level.
When the output frequency f is lowered (at p1) using a deceleration command and the motor is decelerated, the DC bus voltage Vdc is raised by the regenerated energy from the motor. When the DC bus voltage Vdc becomes the DC bus voltage stalling level Vdc0 or more (at p2 and p4), the output frequency f is raised to prevent an overvoltage trip. Since the output frequency f is raised, the regenerated energy is reduced; when the DC bus voltage Vdc is lowered and becomes the DC bus voltage stalling level Vdc0 or less (at p3 and p5), the deceleration command is executed again.
In the operation methods for the conventional inverter controllers during an instantaneous power failure, the motor is driven for regenerative operation by adjusting a speed command or by determining a deceleration rate on the basis of the level of the terminal voltage (intermediate voltage) of a smoothing capacitor or the operation of a power failure detection circuit.
Furthermore, in the operation methods for the conventional inverter controllers at the time when the DC bus voltage is raised to a predetermined voltage or more by the regenerated energy during the deceleration of the AC motor and the operation of the AC motor under a regenerative load, the step of lengthening the deceleration time, stopping the deceleration or raising the output frequency depending on the level of the DC bus voltage is repeated to suppress the DC bus voltage from rising, whereby an overvoltage trip is prevented.
Patent document 1: Unexamined Japanese Patent Application Publication No. Hei 6-165579
Patent document 2: Japanese Patent No. 3201460
Patent document 3: Japanese Patent No. 3095083
Patent document 4: Japanese Patent No. 3536695